import React, { useState } from 'react'
import { useEffect } from 'react'

export default function App() {
  let [opacity, setOpacity] = useState(1);
  
  // 初始化的时候添加一个定时器
  
  useEffect(() => {
    const opacityTimer = setInterval(() => {
      opacity -= 0.1;
      if(opacity <= 0) {
        opacity = 1;
      }
      setOpacity(opacity);
    },100);

    return () => {
      clearInterval(opacityTimer);
    }
  }, []);


 
  return (
    <div>
    <h1 style={{opacity}}>ReactHook 真好用</h1>
    
    </div>
  )
}
 